Sensing devices for determining an electrical conductivity and temperature can be used for the monitoring of (dish)washing processes in order to e.g. determine the concentration of detergent available for the (dish)washing process and to re-dose (fill) detergent if required. Although the electrical conductivity is a measure for the concentration of detergent, the temperature is measured as well, since the electrical conductivity is temperature-dependant.
U.S. Pat. No. 4,733,798 describes a method and apparatus for controlling the concentration of wash water in a ware washing machine, in which the conductivity of the ware washing solution is measured, as well as the temperature, in order to compensate for the apparent concentration changes solely associated with changes in temperature of the washing solution.
EP 1 704 810, in the name of the present applicant, describes a self-contained and wireless monitoring device, e.g., for monitoring a washing process inside a relatively small industrial dishwashing machine, which device monitors the electrical conductivity and temperature of the washing liquid. The temperature sensor in the monitoring device is physically isolated from the liquid in the sense of being encapsulated by a material that protects the sensor against the harsh chemical environment wherein the monitoring device operates during dishwashing. The disclosed monitoring device uses a stored threshold value of the electrical conductivity below which the detergent concentration in the washing liquid is considered too low and a user is alerted.
A problem in determining the amount of detergent by measuring the electrical conductivity and temperature, is that the quality, in particular the electrical conductivity, of water without detergent varies from one geographical region to another. This variation may be larger than the influence of the addition of detergent. In order to compensate for this variation to determine a reliable threshold value, it is desirable to obtain information on the electrical conductivity and the temperature at which that electrical conductivity of the washing liquid without dissolved detergent was determined. Users of the sensing devices, however, frequently immerse the sensing device in the washing liquid almost simultaneously with adding the detergent to the liquid. Whereas the electrical conductivity of the water can be measured quickly, the associated temperature cannot as a result of the physical encapsulation of the temperature sensor in the sensing device.
Therefore, a need exists in the art for a sensing device that is capable of quickly and accurately determining a temperature of a liquid and an electrical conductivity at that temperature.